1. Field of the Invention
The present invention relates to communication technology and especially relates to a radio apparatus mounted on a vehicle.
2. Description of the Related Art
In order to improve safety in driving of the vehicle, technology to transmit alert information to another vehicle using a radio signal is used. A driver of the other vehicle recognizes an approach of the vehicle based on the alert information. For example, when the driver of a certain vehicle inputs the alert information, a transmitting apparatus transmits the alert information to which a current position is added.
When a receiving apparatus receives the alert information, this calculates a distance between the vehicles based on the current position and outputs an alert signal from a speaker only when the distance is not larger than a predetermined distance.
A collision accident between the vehicles is likely to occur at an intersection. Therefore, it is especially important at the intersection to use a radio signal in order to notify another vehicle of presence of its own vehicle. When the number of vehicles, which travel in the vicinity of the intersection, increases, the number of transmitted radio signals also increases. As a result, collision probability between the radio signals increases and the number of radio signals received by the receiving apparatus also increases. In the former case, the radio signal does not arrive at the receiving apparatus, so that alert to another vehicle is not performed. In the latter case, notification is continuously performed, so that the driver is notable to recognize the vehicle to which the driver should pay attention the most. On the other hand, there is a case in which a traffic signal is provided and a case in which the traffic signal is not provided at the intersection. In general, a degree of risk is higher in the latter case than in the former case. Therefore, alert by the radio signal is especially effective in the latter case.